In its simplest form, a copper-clad laminate includes, as a component, one or more dielectric layers of woven or nonwoven glass fiber impregnated with epoxy or other polymer resin, the composite is known in the art as “prepreg.” The prepreg is sandwiched between sheets of metal foil, typically copper, and supported by a steel or aluminum plate of varying thickness. These plates act as a tooling plate for subsequent high-pressure lamination, which bonds the copper foil and epoxy prepreg, resulting in a laminate sheet.
These laminates are generally manufactured in bulk and are generally stacked one upon another. The assembly of one laminate is referred to as a press layup and the stack of laminates is known as a book. Presently, the book is manufactured by a process involving heating a plurality of layers that make up each laminate and then subjecting the book to pressure. After curing and cooling, the then-bonded individual laminates are separated from one another and subjected to further processing.
It is typically highly desirable in the manufacture of these laminates to prevent and eliminate, to the greatest practicable extent, contamination of the foil sheets and to maintain cleanliness during the manufacturing process. It is known that the main causes of contamination are the presence of resin dust, fiberglass fibers, particulate matter and various other types of foreign material. During the manufacturing process, great care is taken to prevent resin dust and other contaminants from settling on the copper surfaces.
The presence of pits or dents or unwanted deposits of molten resolidified resin on the surface of the copper sheets is a major cause for concern in the manufacturing process. These pits and dents can result from the presence of resin dust on the foil during the heating and laminating process, which in turn causes an indentation or depression in the copper. Pits and/or dents in the copper foil may also result from the use and handling of extremely thin foils. The presence of a pit, dent or unwanted deposit of molten resolidified resin on the surface of the copper foil will generally result in a defect in the finished product due to a shorted or open conductive path in an etched circuit layer.
In one known method for manufacturing laminates for printed circuit boards, a laminate of a sheet of copper foil and a sheet of aluminum or the like are joined around their borders using a band of flexible adhesive, creating a protected central zone at the interface of the sheets. One drawback to this method, however, is that the use of adhesive in the manufacture of the laminate renders the surface susceptible to the aforementioned problems involving the presence of contaminants and stains, pits and/or unwanted deposits of dried adhesive. Additionally, the adhesive bond has been known to induce stress that can result in wrinkles along one or more borders in the finished laminate.
In another known method, a strip of copper foil is ultrasonically welded to a second strip of a supporting metal, preferably aluminum or stainless steel, at their respective edges. The ultrasonic welding process, while effective in bonding the sheets of copper and aluminum, is slow and requires multiple machines to match the current productivity rate compared to the typical steel plate manufacturing method.
In both of these methods, the actual construction of an electrical grade laminate becomes a secondary operation, making the process more costly due to the additional handling, packaging shipping and assembly steps required. Additionally, the user is constrained by limited supply sources and requires excess inventories on hand to meet ever-changing market and customer demands.
The conductive copper foils that are used in the production of laminates generally have an untreated surface on one side and a treated surface on the opposite side. The treatment that is performed on the “treated” side typically includes plating the copper surface with copper nodules to promote adhesion. In the previously described copper/protective carrier/copper component, the untreated surfaces of the copper sheets face and contact the protective carrier layer disposed between the copper foil sheets. In a printed circuit board laminate, the untreated surfaces of the copper sheets constitute a functional element and are generally etched to produce the desired circuit conductor configuration. It is for this reason that the untreated surfaces of the copper foil sheets should be kept substantially uncontaminated and free of imperfections. The treated surfaces of the copper foil sheets contact and adhere to the prepregs in the book layup.